Optical fluorophosphate glass with anomalous positive incremental dispersion and process for making it

ABSTRACT

An optical fluorophosphate glass with an anomalous positive incremental dispersion +Δν e  between 11.8 and 12.5, a refractive index n e  between 1.53 and 1.55 and an Abbe coefficient ν e  between 72.8 and 73.5 has the following atomic composition (in atom %): Mg: 2.0-3.5, Ca: 2.5-4.5, Sr: 13.0-16.0, Ba: 22.0-25.0, Al: 4.5-6.0, K: 0.1-0.3, P: 9.0-12.0, O: 17.0-20.0 and F: 20.0-23.0, the F/P ratio lying between 1.8 and 2.4 and the sum of the alkaline earths between 39.5 and 49. The new glasses are melted from a material having the following initial composition (in wt. %): Ca(PO 3 ) 2  : 1.0-2.5, Ba(PO 3 ) 2  : 23.0-25.5, Al(PO 3 ) 3  : 12.0-14.5, MgF 2  : 6.5-8.5, CaF 2  : 5.5-8.0, SrF 2  : 16.0-19.0, AlF 3  : 10.0-12.5, SrO: 2.0-3.0, BaO: 12.5-14.5 and K 2  TiF 6  : 0.2-0.7, the alkaline earth fluorides content lying between 28.0 and 35.5 and the total content of all fluorides used between 38.2 and 48.7.

The application relates to an optical fluorophosphate glass havinganomalous positive partial dispersion, in which the tendency towardcrystallization is extremely low.

An optical glass based on metaphosphates and fluorides of alkaline-earthmetals and aluminum with the following composition is already known:

    ______________________________________                                        Ca(PO.sub.3).sub.2 :                                                                            2.53%   by weight                                           Ba(PO.sub.3).sub.2 :                                                                            18.01%  by weight                                           Al(PO.sub.3).sub.3 :                                                                            22.94%  by weight                                           MgF.sub.2 :       9.70%   by weight                                           CaF.sub.2 :       7.40%   by weight                                           SrF.sub.2 :       21.80%  by weight                                           AlF.sub.3 :       6.69%   by weight                                           BaO:              9.93%   by weight                                           KHF.sub.2 :       1.00%   by weight                                           ______________________________________                                    

The elemental composition of this known glass is as follows (inweight-%):

Mg: 3.78; Ca: 4.31; Sr: 15.20; Ba: 17.29; Al: 4.49;

K: 0.50; P: 12.66; 0: 20.62 and F: 21.15.

This known glass has a refractive index n_(e) of 1.544 and an Abbenumber ν_(e) of 73.1. However, melting is only possible with greattechnological difficulty. The yield is extremely low, due, inter alia,to the high tendency of this glass towards crystallization.

It is therefore an object of the present invention to provide a glasswith analogous optical properties, i.e. with an identical refractiveindex n_(e) and an identical Abbe number ν_(e), in which the abovementioned technological difficulties in melting are avoided. It is afurther object to indicate a process for crystallization-free melting ofthe glass in large units.

This object is achieved according to the invention by providing afluorophosphate optical glass produced by melting a mixture having thefollowing composition (in % by weight):

    ______________________________________                                               Ca(PO.sub.3).sub.2                                                                   1.0-2.5                                                                Ba(PO.sub.3).sub.2                                                                   23.0-25.5                                                              Al(PO.sub.3).sub.3                                                                   12.0-14.5                                                              MgF.sub.2                                                                            6.5-8.5                                                                CaF.sub.2                                                                            5.5-8.0                                                                SrF.sub.2                                                                            16.0-19.0                                                              AlF.sub.3                                                                            10.0-12.5                                                              SrO    2.0-3.0                                                                BaO    12.5-14.5                                                              K.sub.2 TiF.sub.6                                                                    0.2-0.7                                                         ______________________________________                                    

wherein the alkaline earth metal fluoride content is between 28 and35.5% by weight; the total content of all fluorides is between 38.2 and48.7% by weight, and the F/P ratio is between 1.8 and 2.4; said glasshaving a refractive index n_(e) between 1.53 and 1.55, an Abbe numberν_(e) between 72.8 and 73.5, and an anomalous positive partialdispersion Δν_(e) between +11.8 and +12.5.

The glasses according to the invention have the following elementalcomposition: (in wt. %)

                  TABLE 1                                                         ______________________________________                                        (in atom-%):                                                                  Range of the     Preferred                                                                              Preferred single                                    invention        range    example                                             ______________________________________                                        Mg     2.0-3.5       2.0-3.0   2.93                                           Ca     2.4-4.5       3.0-4.0   3.88                                           Sr     13.0-16.0     14.0-15.0                                                                              14.51                                           Ba     22.0-25.0     23.0-24.0                                                                              23.55                                           Al     4.5-6.0       5.0-6.0   5.05                                           K      0.1-0.3       0.1-0.2   0.16                                           P       9.0-12.0     10.0-11.0                                                                              10.51                                           O      17.0-20.0     18.0-19.0                                                                              18.14                                           F      20.0-23.0     21.0-22.0                                                                              21.27                                           n.sub.e                                                                              1.53-1.55     1.54-1.55                                                                               1.544                                          ν.sub.e                                                                           72.8-73.5     73.0-73.2                                                                              73.10                                           Δν.sub.e                                                                    11.8-12.5     12.1-12.2                                                                              12.17                                           ______________________________________                                    

In Table 2 below, the ranges of the invention are indicated for theinitial compositions: (in wt. %)

                  TABLE 2                                                         ______________________________________                                        (in % by weight):                                                                               Preferred  Preferred                                               Range of the                                                                             interval of                                                                              individual                                              invention  the invention                                                                            example                                          ______________________________________                                        Ca(PO.sub.3).sub.2                                                                     1.0-2.5      1.6-1.8     1.7                                         Ba(PO.sub.3).sub.2                                                                     23.0-25.5    24.0-25.0  24.2                                         Al(PO.sub.3).sub.3                                                                     12.0-14.5    13.0-14.0  13.9                                         MgF.sub.2                                                                              6.5-8.5      7.0-8.0     7.5                                         CaF.sub.2                                                                              5.5-8.0      6.0-7.0     6.9                                         SrF.sub.2                                                                              16.0-19.0    17.0-18.0  17.9                                         AlF.sub.3                                                                              10.0-12.5    11.0-12.0  11.3                                         SrO      2.0-3.0      2.3-2.5     2.4                                         BaO      12.5-14.5    13.0-14.0  13.7                                         K.sub.2 TiF.sub.6                                                                      0.2-0.7      0.4-0.6     0.5                                         ______________________________________                                    

The initial quantity is molten in a manner known per se in a platinumcrucible. The glass molten from the initial quantity is colorless,ream-free, stable to crystallization and can be produced insignificantly higher yield than the known glass. After slow cooling, theindividual example shown has the following optical data:

n_(e) =1.544

ν_(e) =73.10

θ'_(g) =0.4804

+Δν_(e) =12.17

The symbols here have the following meanings:

n_(e) =refractive index

ν_(e) =Abbe number (reciprocal of the dispersion)

θ'_(g) =(actual) anomalous partial dispersion, where the followingapplies: ##EQU1## In this equation, the subscripts have the followingmeanings: g=blue mercury line (435.84 nm);

F'=blue cadmium line (479.99 nm);

C'=red cadmium line (643.85 nm);

+Δν_(e) =positive deviation from the so-called "standard straight line",as shown in diagrammatic form and described in greater detail in, forexample, the drawing of German Patent 1,496,563 and corresponding U.S.Pat. No. 3,451,829. This difference is known as the "positive anomalouspartial dispersion value".

The glass obtained has the transmission characteristics shown in Table 3below:

                  TABLE 3                                                         ______________________________________                                        λ[nm]   τ;[5nm]                                                                           τ;[25nm]                                           ______________________________________                                         1014.0        0.998   0.990                                                  700            0.999   0.995                                                  660            0.999   0.995                                                  620            0.999   0.995                                                  580            0.998   0.993                                                    546.1        0.999   0.997                                                  500            0.997   0.988                                                  460            0.996   0.983                                                    435.8        0.994   0.973                                                  420            0.993   0.968                                                    404.7        0.992   0.963                                                  400            0.991   0.958                                                  390            0.989   0.948                                                  380            0.985   0.927                                                  370            0.976   0.884                                                  365            0.963   0.829                                                  350            0.883   0.537                                                    334.1        0.612   0.086                                                  320            0.158   --                                                     310            0.045   --                                                     ______________________________________                                    

In this table, the symbols have the following meanings:

λ: the measurement wavelength used [nm];

τ_(i) (5 mm): the net transmittance of a glass plate of thickness 5 mm;

τ_(i) (25 mm): the net transmittance of a glass plate of thickness 25mm.

Further physico-chemical properties of the glass obtained are shownbelow:

linear thermal expansion coefficient: α_(20/300)° C. =13.43·10⁻⁶ /K;

Transformation temperature T_(g) =502° C.;

Density ρ=3.64 g/cm³ ;

Knoop hardness HK=408.

The glass according to the invention can be press-molded and islow-fluorescence and can be used for highly corrected objectives.

We claim:
 1. Optical fluorophosphate glass produced by melting a mixturehaving the following composition (in % by weight):

    ______________________________________                                               Ca(PO.sub.3).sub.2                                                                   1.0-2.5                                                                Ba(PO.sub.3).sub.2                                                                   23.0-25.5                                                              Al(PO.sub.3).sub.3                                                                   12.0-14.5                                                              MgF.sub.2                                                                            6.5-8.5                                                                CaF.sub.2                                                                            5.5-8.0                                                                SrF.sub.2                                                                            16.0-19.0                                                              AlF.sub.3                                                                            10.0-12.5                                                              SrO    2.0-3.0                                                                BaO    12.5-14.5                                                              K.sub.2 TiF.sub.6                                                                    0.2-0.7                                                         ______________________________________                                    

wherein the alkaline earth metal fluoride content is between 28 and35.5% by weight; the total content of all fluorides is between 38.2 and48.7% by weight, and the F/P ratio is between 1.8 and 2.4; said glasshaving a refractive index n_(e) between 1.53 and 1.55, an Abbe numberν_(e) between 72.8 and 73.5, and an anomalous positive partialdispersion Δν_(e) between +11.8 and +12.5.
 2. Glass according to claim1, wherein said glass is produced by melting a mixture having thefollowing composition (in % by weight):

    ______________________________________                                               Ca(PO.sub.3).sub.2                                                                   1.6-1.8                                                                Ba(PO.sub.3).sub.2                                                                   24.0-25.0                                                              Al(PO.sub.3).sub.3                                                                   13.0-14.0                                                              MgF.sub.2                                                                            7.0-8.0                                                                CaF.sub.2                                                                            6.0-7.0                                                                SrF.sub.2                                                                            17.0-18.0                                                              AlF.sub.3                                                                            11.0-12.0                                                              SrO    2.3-2.5                                                                BaO    13.0-14.0                                                              K.sub.2 TiF.sub.6                                                                    0.4-0.6                                                         ______________________________________                                    

wherein the alkaline earth metal fluoride content is between 30 and 33%by weight; the total content of all fluorides is between 41.4 and 45.6%by weight, and the F/P ratio is between 1.9 and 2.2; said glass having arefractive index n_(e) between 1.54 and 1.55, an Abbe number ν_(e)between 73.0 and 73.2, and an anomalous positive partial dispersionΔν_(e) between +12.1 and +12.2.
 3. Glass according to claim 2, whereinsaid glass is produced by melting a mixture having the followingcomposition (in % by weight):

    ______________________________________                                                Ca(PO.sub.3).sub.2                                                                    1.7                                                                   Ba(PO.sub.3).sub.2                                                                   24.2                                                                   Al(PO.sub.3).sub.3                                                                   13.9                                                                   MgF.sub.2                                                                             7.5                                                                   CaF.sub.2                                                                             6.9                                                                   SrF.sub.2                                                                            17.9                                                                   AlF.sub.3                                                                            11.3                                                                   SrO     2.4                                                                   BaO    13.7                                                                   K.sub.2 TiF.sub.6                                                                     0.5                                                           ______________________________________                                    

wherein the alkaline earth metal fluoride content is 32.3% by weight;the total content of all fluorides is 44.1% by weight; and the F/P ratiois 2.0; said glass having a refractive index n_(e) of 1.544, an Abbenumber ν_(e) of 73.1, and an anomalous positive partial dispersionΔν_(e) of +12.17.
 4. Glass according to claim 3, wherein said glass hasthe following elemental composition (in % by weight):

    ______________________________________                                                Mg    2.93                                                                    Ca    3.88                                                                    Sr   14.51                                                                    Ba   23.55                                                                    Al    5.05                                                                    K     0.16                                                                    P    10.51                                                                    O    18.14                                                                    F     21.27.                                                          ______________________________________                                    


5. Glass according to claim 4 having a linear thermal expansioncoefficient α_(20/300)° C. of 13.43·10⁻⁶ /K, a transformationtemperature T_(g) of 502° C., a density of 3.64 g/cm³, and a KnoopHardness HK of 408.